What Is the Difference Between a Flash Point and an Autoignition Temperature?

Introduction

The flash point of a fuel is the temperature at which it gives off enough flammable vapours to ignite when a source of ignition, like an open flame, is introduced. This should not be confused with the fire point, which is the temperature at which the vapours of the fuel will continue to burn once the source of ignition is removed, usually for a minimum of five seconds. A third point, the autoignition temperature, should also not be confused with the first two. The autoignition temperature is the temperature at which the fuel will spontaneously ignite without the need for a spark, flame, or any other ignition source.

Understanding flash points and auto-ignition temperatures is vital if you work in the fuel industry, or in an industry that utilises industrial amounts of fuel. This is because some fuels – like kerosene – have a flash point that’s within the normal temperature ranges of some places on earth. This means, if you live in a hot region like Australia, North Africa, or the Middle-East, kerosene fuel may be more volatile and will ignite far more easily than it would in Northern Europe, North America, or Russia.

Let’s take a look at some of the common fuels in the industry and what their flash points and autoignition temperatures are:

Ethanol (70%)

Ethanol of 70% purity has a flashpoint of around 16-17°C (61.9 °F), meaning that, in most habitable places in the world, ethanol will be above its flashpoint for most of the year, dropping below it only during the colder winter months. Ethanol’s high autoignition temperature at 363 °C (685 °F) means it cannot auto ignite anywhere on earth without being artificially heated.

Petrol and Diesel

Petrol (gasoline) has a fairly low flashpoint at −43 °C (−45 °F). This means that, unless you’re in one of the coldest places in the world, your petrol fuel is above its flashpoint. White Diesel, on the other hand, enjoys a mid-range flash point of over 52 °C (126 °F), meaning the exact opposite! Diesel will likely be below its flash point unless you are in a particularly hot place.

Both petrol and diesel have quite high autoignition temperatures, or our vehicles would be in trouble! Petrol and diesel have autoignition temperatures of 280 °C (536 °F) and 210 °C (410 °F), respectively.

Kerosene

Kerosene has a broad flashpoint that can be as low as 36 °C (99 °F) and as high as 72 °C (162 °F), meaning it requires a higher degree of care when it comes to storage and operation temperatures interacting with its flashpoint. It does, however, have a similar autoignition temperature to petrol and diesel at 220 °C (428 °F).

 

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